1. Field of the Invention
The present invention relates to a measuring attachment connectible to a multimeter for measuring earth resistances, of the type comprising a casing having a first group of terminals for a measurement signal and a second group of terminals for the electrical connection of the attachment to the multimeter, and electric circuits housed in the casing and connected electrically to the terminals of the first and second groups of terminals.
2. Description of the Prior Art.
Grounding or earthing of the masses of household and industrial electric appliances is required by regulations for ensuring the safety of the users of such appliances.
The efficiency of this earthing is considered satisfactory if the voltage to which the accessible metal parts (mass) are raised does not exceed a certain threshold U.sub.s (generally fixed at 24 V) should there be a defect in the installation. Since the defect currents themselves are limited by the value of the current I.sub.d for tripping the protection apparatuses, it is clear that the efficiency of earthing will be related to not overshooting a resistance value defined by the quotient U.sub.s /I.sub.d.
So it is necessary, to satisfy the requirements of safety regulations, to measure the resistance of earth tappings not only at the time of initial installation, but also subsequently so as to be sure that they maintain their quality.
This measurement however requires, to be significant, to be carried out according to an appropriate procedure;
(1) Using an AC current for eliminating the effect of biasing voltages.
(2) Choosing for this current a frequency different from that of the mains and its harmonics.
(3) Providing effective filtering of parasitic AC and DC voltages to bring them to a level which does not disturb the measurement.
These requirements have led to the construction of measuring apparatus specialized in measuring earth resistances and relatively cumbersome.
On the other hand digital or analog multimeters are known, to which various attachments may be connected for increasing the number of ranges and/or functions of the multimeter. Such multimeters are, for example commercialized by the firm Chauvin Arnoux under the names "CONPA 2010", "CONPA 2011" and "CONTA 20011". With these multimeters, the firm Chauvin Arnoux commercializes a number of plug-in attachments which, when they are connected to the multimeter confer thereon the following functions: thermometer, luxmeter/luminance meter, frequency meter, voltage or current generator, capacitance meter sonometer, ammeter for measuring high value AC or DC currents. However, to the knowledge of the Applicant, an attachment has not yet been proposed to be connected to a multimeter for measuring earth resistances with the multimeter.